The evolution of embryological development has long been characterized by deep conservation. In animal development, the phylotypic stage in mid-embryogenesis is more conserved than either early or late stages among species within the same phylum. Hypotheses to explain this hourglass pattern have focused on purifying the selection of gene regulation. Here, we propose an alternative—genes are regulated in different ways at different stages and have different intrinsic capacities to respond to perturbations on gene expression. To eliminate the influence of natural selection, we quantified the expression variability of isogenetic single embryo transcriptomes throughout fly Drosophila melanogaster embryogenesis. We found that the expression variability is lower at the phylotypic stage, supporting that the underlying regulatory architecture in this stage is more robust to stochastic variation on gene expression. We present evidence that the phylotypic stage is also robust to genetic variations on gene expression. Moreover, chromatin regulation appears to play a key role in the variation and evolution of gene expression. We suggest that a phylum-level pattern of embryonic conservation can be explained by the intrinsic difference of gene regulatory mechanisms in different stages.

中文翻译：

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胚胎间基因表达差异概括了evo-devo的沙漏模​​式。

长期以来，胚胎发育的发展一直以深度保护为特征。在动物发育中，同一个门脉内的物种在胚胎发生中期的系统分型阶段比早期或后期更为保守。解释这种沙漏模式的假设集中在纯化基因调控的选择上。在这里，我们提出了一种替代方案-基因在不同阶段以不同的方式受到调节，并且具有不同的内在能力来响应基因表达的扰动。为了消除自然选择的影响，我们量化了整个果蝇果蝇胚胎发生过程中同基因单胚转录组的表达变异性。我们发现系统发育阶段的表达变异性较低，支持这一阶段的基础调控体系结构对基因表达的随机变异更为稳健。我们目前的证据表明，系统发育阶段也对基因表达的遗传变异具有鲁棒性。此外，染色质调节似乎在基因表达的变异和进化中起关键作用。我们建议，可以通过不同阶段的基因调控机制的内在差异来解释门级的胚胎保护模式。